Technical Field of the Invention
The invention relates to a method for coating the surface of inorganic solid particles in an aqueous suspension. It particularly relates to the coating of titanium dioxide pigment particles with silicon dioxide and aluminum oxide.
Description of Related Art
The surface of fine, inorganic solid particles is frequently coated in order to modify certain properties, such as abrasion resistance, surface charge, dispersing properties, acid or light resistance. For example, U.S. Pat. No. 2,886,366 describes the application of a dense silicon dioxide coating to such substrate particles as nickel or iron powder, glass fibers or titanium dioxide. Color and white pigments are regularly coated with various oxides and hydroxides (e.g. EP 0 130 272 A1, U.S. Re. 27818).
Surface treatment (post-treatment), particularly of TiO2 pigments, is customarily performed in the aqueous phase, where metal oxides, hydroxides, phosphates or similar compounds are deposited on the particle surface. The method is customarily implemented in the form of a batch process in an agitated tank, as disclosed in GB 1 340 045, for example. Starting with an aqueous pigment particle suspension, corresponding metal salts are added in dissolved form as so-called precursor compounds. Alkaline or acid substances are then used to set the pH value of the suspension in such a way that the precursor compounds are precipitated in the form of oxides, hydroxides, etc.
For example, GB 1 340 045 discloses a batch process for coating the surface of titanium dioxide pigment, where the pigment, in the form of a suspension, is subjected to intensive stirring in an agitated vessel for up to two hours, during which time the coating substances are added and precipitated.
However, owing to the relatively large volume of the agitated tank and the limited possibilities for agitation, local concentration, pH, viscosity and temperature gradients occur during addition of the precursor compounds, these impairing the result of the coating of titanium dioxide particles. The resultant coating is of inconsistent density and/or inconsistent thickness.
Moreover, particle agglomeration can easily occur in the suspension, such that the deposited coating substances envelope not an individual particle, but frequently an agglomerate. The agglomerates are broken up again during subsequent dry milling, meaning that not all the particles in the end product are provided with a closed skin, but that the particles also display uncoated surface areas. In addition, part of the coating substances is not fixed on the particle surface, but forms flocs alongside the particles. These flocs can no longer be removed from the suspension and have a negative impact on the optical properties of the pigments, e.g. on the tinting strength (TS).
One advantage of the customary batch process in an agitated tank is that the dwell time is adapted to the reaction kinetics. In contrast, a disadvantage is that temperature profiles cannot be realized during the coating process without unreasonable effort.
An improvement of the known batch process for coating titanium dioxide in an agitated tank is the use of a cascade of agitated tanks. In this case, the precursor compounds of the inorganic oxides are each added in a separate, smaller agitated tank with improved possibilities for agitation.
U.S. Pat. No. 6,695,906 B2 describes a method of this kind, where TiO2 pigment is coated with SiO2 and Al2O3. The soluble SiO2 component is added in three steps in three consecutive treatment vessels, in each of which a different pH value occurs. In this context, the SiO2 component is added either directly to the treatment vessel, or inline to the feed line.
Nevertheless, local concentration, pH, viscosity and temperature gradients also occur in this case, leading to inconsistently dense and/or inconsistently thick coating of the titanium dioxide particles. The disadvantage likewise remains that temperature profiles are hard to realize.
A second improvement of the known batch process for coating titanium dioxide in an agitated tank is a continuous process in a tubular reactor. In this case, the precursor compounds of the inorganic oxides are each added to a tubular reactor with ideal possibilities for agitation (inline mixer or inline disperser). The precursor compounds are added at the molecular level, as it were, i.e. the precursor compounds to be reacted and the titanium dioxide particles to be coated are brought together in a very confined space and ideally mixed. This avoids local concentration, pH, viscosity and temperature gradients.
A method of this kind is disclosed in U.S. Pat. No. 5,993,533, for example, where titanium dioxide pigment is coated with SiO2 and Al2O3 in two consecutive curing stages. The precursor compound is added to the TiO2 slurry in an inline mixer. The slurry is subsequently fed into a tank and the coating substance is precipitated at 80 to 100° C. and pH values of >9.5 and <8.5, respectively.
US 2009/0297852 A1 likewise describes a continuous process for coating inorganic particles, where the precursor compounds are added to the slurry directly upstream of, or in, an inline disperser and the slurry is homogenized. The slurry is subsequently fed into a tank and the coating substance is precipitated.
DE10 2006 059 849 A1 discloses a coating method, where the precursor substances is added to the slurry directly upstream of, or in, an agitator mill and precipitation of the coating substance subsequently takes place in a tank.
The continuous process permits the realization of temperature profiles. However, it has the disadvantage that a dwell time adapted to the reaction kinetics can only be realized either by means of a possibly very long tube, avoiding the development of a laminar flow profile, or by means of a correspondingly large cure tank. Another disadvantage of the continuous process is that, if process control is not ideal, it is impossible to rule out the possibility of mixing of the individual inorganic oxides with the coating materials subsequently applied, this producing mixed-oxide layers instead of separate oxide layers.